In the production of phosphorus trichloride raw material impurities accumulate in the chlorination reactor and it is necessary to periodically remove such impurities by terminating the reaction and flushing out the reactor with water. The reaction, as generally carried out, involves the introduction of elemental phosphorus into a reactor which contains phosphorus trichloride. A separate liquid layer of elemental phosphorus may be established below the phosphorus trichloride. Chlorine gas is introduced into the phosphorus trichloride by means of spargers which agitate the contents of the reactor and assist in dissolving phosphorus in the liquid layer. The dissolved phosphorus in the phosphorus trichloride layer reacts with the chlorine producing phosphorus trichloride. Because the reaction is exothermic phosphorous trichloride is caused to vaporize and to be taken out of the reactor overhead where it is condensed and purified. It has been found that raw material impurities (mostly from elemental phosphorus) accumulate in the chlorination reactor and must be periodically removed.
The procedure for periodic removal of impurities from the reactor comprises first terminating the introduction of phosphorus into the reactor. The residual phosphorus is then consumed by reaction with chlorine. The next important decision to be made in terminating the reaction is when to terminate the chlorine feed to the reactor. If the chlorine feed is terminated too early small amounts of elemental phosphorus may remain in the reactor and cause a dangerous situation when the reactor is opened for cleaning. If the chlorination continues for a period of time subsequent to the consumption of all of the elemental phosphorus in the reactor excessive amounts of phosphorus pentachloride are produced thereby creating a waste disposal problem.
One of the common impurities in phosphorus is water which is used to protect phosphorus from contact with air to cause spontaneous combustion. Water will react with phosphorus trichloride to form phosphorus acid, phosphoric acid and many undefined lower oxides mixtures. These lower oxides must be chlorinated to form either volatile chlorides or oxidized to stable phosphoric acid. To determine the end point of chlorination of the lower oxides is difficult and is largely unknown.
In the past various means have been taken to determine when the chlorine feed to the reactor should be terminated. Rapid, periodic analysis of samples from the reactor indicating the amount of phosphorus in the reactor has been found to be dangerous, cumbersome and unreliable. One such method is disclosed in Chem. Tech (Leipzig) Vol. 24, No. 6, page 363 (1972) by Rudolph Schumann. In this method a measured sample of the phosphorus trichloride solution is withdrawn from the reactor and poured into a Dewar flask containing bromine dissolved in carbon tetrachloride. The temperature of the solution before addition of the sample is noted and after thorough mixing the temperature is again noted. The temperature difference is a rough measure of the phosphorus concentration as determined by comparison with a calibration curve.
Another method of analysis for phosphorus and phosphorus chlorides is gas chromatography. Such a method is reported by N. M. Kirievskaya et al in a publication entitled "Gas-Chromatographic Determination of White Phosphorus and of Certain Phosphorus Chlorides" appearing in Zhurnal's Prikladnoi Khimii, Vol. 45, No. 9, pages 2074-2076, (1972). A chromatograph was employed to determine white phosphorus and various phosphorus chlorides with a 5,000.times.3 mn column packed with Celite 545 and SKT-FT-100.times.methylfluoropropylsilicone rubber. The column temperature was 150.degree. C. and a thermal conductivity detector was used. The method was based upon mixtures of reagent grade materials mixed together in the laboratory.
Various measurements of reactor conditions have been employed to aid in the determination of heel chlorination end point. An example is provided by Japanese Patent Publication 54-16957 to Zinno et al wherein the temperature of the cooling solvent employed to control the temperature of the reactor was used to determine the completion of the reaction of yellow phosphorus with chlorine within the reactor. According to this publication the reaction of yellow phosphorus and chlorine was controlled to a temperature within 1.degree. C. of a specified temperature. After the supply of yellow phosphorus to the reactor is terminated the chlorine feed is continually supplied to the reactor until the temperature at the exit of the cooling solvent condenser, or the reaction tower top or the reactor interior decreases 2.degree. C. below the reaction temperature. The temperature drop was considered an indication of the termination of the exothermic reaction of yellow phosphorus and chlorine.
Even when, by the various means available, it was believed that all of the phosphorus in the reactor was consumed it has been found that reactive chemicals remained in the reactor which created safety hazards upon opening the reactor for cleaning. A more detailed study of the chemistry of the reaction was needed to determine the source of reactive chemicals remaining in the reactor even after all of the elemental phosphorus was consumed. There was therefore needed a more specific and more consistent means for determining the end point of heel chlorination to provide a safe, efficient and ecologically more acceptable means for operating the process for preparing phosphorus trichloride.